A secondary battery state of charge determination apparatus and a method of determining a state of charge of a secondary battery are for measuring values related to state of charge required for controlling charging and discharging of the secondary battery. The state of charge may be hereinafter referred to as SOC (state of charge).
The SOC a ratio of a remaining capacity of the secondary battery at a specific time point to a remaining capacity of the secondary battery at full charge (i.e., a capacity of the secondary battery).
A method of determining the SOC of a secondary battery may use open-circuit-voltage-to-charging-condition characteristics to determine the SOC based on an open circuit voltage (may be hereinafter referred to as OCV) as disclosed in Patent Document 1. In another method, charge and discharge currents for a secondary battery are accumulated, and a ratio of the accumulated current to a total capacity is calculated as the SOC.
The latter method in which the charge and discharge currents of the secondary battery are accumulated normally uses an initial total capacity as the total capacity of the secondary battery, which is a denominator used in a calculation of the SOC. If the capacity of the battery decreases due to degradation of the secondary battery, the calculated SOC is more likely to include an error.
The former method is widely used former because open-circuit-voltage-to-charging-condition characteristics (may be hereinafter referred to as OCV-SOC characteristics) are considered not to change even if the total capacity of the secondary battery varies due to the degradation thereof.